Field of the Invention
The present invention relates to a technique of driving a lens in a camera system such as a single-lens reflex type digital camera.
Description of the Related Art
Control methods in automatic focus adjustment for image capturing apparatuses are roughly classified into two types. One method is a ONE-SHOT method of shooting a still object, and performing focus lock after focusing by an AF operation. The other method is an AI-SERVO method of shooting a moving object and continuing focus detection periodically and lens driving as needed unless the AF operation is completely turned off.
In general, an AF lens is focused by causing a motor to drive the focus lens. Conventionally, when driving a focus lens, there is a tradeoff relationship between a position stop accuracy and a driving time. That is, when priority is given to the stop position accuracy, the driving time increases, whereas when priority is given to the driving time, the stop position accuracy deteriorates. However, a current camera allows setting of only the driving amount and driving speed of the focus lens.
In a ONE-SHOT operation, higher priority should be given to a stop position accuracy than a driving time unless the shutter has been pressed, because a user presses the shutter upon checking an in-focus state. In an AI-SERVO operation, higher priority should be given to a driving time than a stop position accuracy if the focus lens cannot be driven within a predetermined time in the process of following a moving object.
Japanese Patent Laid-Open No. 2009-128611 discloses a camera which performs moving object prediction control, characterized by comprising a lens driving period prolongation means for setting a predetermined prolonged time if a focus adjustment lens position does not reach a drive target position and lens driving does not stop within the lens driving initial period and a moving object re-prediction computation means, wherein a lens control means controls driving of a focus adjustment lens at a target position calculated by the moving object re-prediction computation means when a driving period is prolonged.
If, however, only the driving amount and driving speed of the focus lens can be set in the camera, the camera cannot know when driving is complete, and cannot hence meet the requirements of the ONE-SHOT mode and the AI-SERVO mode. FIG. 13 graphically represents this state.
In addition, the related art disclosed in Japanese Patent Laid-Open No. 2009-128611 has a problem that the frame rate decreases more and more, and the marginal performance deteriorates.